The invention relates to a process for producing novel attrition-resistant fluidizable petroleum cracking catalysts from zeolite-containing fines produced as a side stream in the manufacture of fluidizable zeolitic cracking catalysts from preformed microspheres of calcined clay(s). In particular, the invention relates to a procedure for making zeolitic cracking catalysts, useful in the fluidized catalytic cracking (FCC) of hydrocarbon feedstocks, from the nominally minus 20 micron sized fine particles generated in practice of "in situ" catalyst manufacturing technology such as described in U.S. Pat. No. 3,647,718.
Crystalline zeolitic molecular sieves are used in a wide variety of catalytic and adsorptive applications. Sieves of the faujasite type, especially ion-exchanged forms of zeolite Y, are well-known constituents of hydrocarbon conversion catalysts. In commercial practice, synthetic forms of zeolite Y, in particular forms of zeolite Y having a SIO.sub.2 /Al.sub.2 O.sub.3 molar ratio above about 4, are utilized as a component of such catalysts.
Synthetic zeolites of the Y-type are commercially available as finely divided high purity crystals. Present commercial use of such zeolites in the fluidized cracking of hydrocarbons requires that the zeolite crystals in the particles of catalyst be associated with a suitable material, sometimes referred to as the matrix, such as a silica-alumina gel, clay or mixture thereof, to provide catalyst particles which operate at activity levels useful in present day cracking units. The matrix material also functions to impart attrition resistance to the catalyst particles, and is a heat transfer medium between the reaction and regeneration sections of the fluid catalytic cracking unit. It is well known that the pore structure and chemical composition of the matrix may have a significant effect on the activity and selectrivity of the catalyst. When preparing cracking catalysts from fine particle size crystals, the choice of a matrix is limited by the fact that the matrix must be thermally stable, provide access of gases or liquids to the zeolite crystals in the composite particles and result in particles of acceptable resistance to attrition.
The synthesis of a variety of zeolites from calcined clays, especially kaolin clay, is known. For example, metakaolin (kaolin clay calcined at a temperature of about 1200.degree. to 1500.degree. F.) will react with sodium hydroxide solution to produce sodium zeolite A. On the other hand, metakaolin can react with sodium silicate solutions under selected conditions to form synthetic zeolites of the faujasite type (so-called zeolite X and zeolite Y). When kaolin is calcined under more severe conditions, sufficient to undergo the characteristic exothermic reation (for example calcination at about 1700.degree. to 2000.degree. F.), the calcined clay will react with sodium hydroxide solution under controlled conditions to synthesize faujasite-type zeolites.
The reaction between kaolin calcined to undergo the exotherm and sodium hydroxide in an aqueous reaction medium is quite sensitive to the history of the clay prior to and during calcination. It is known that the addition of a minor amount of metakaolin relative to kaolin calcined to undergo the exotherm frequently assures that a desired amount of synthetic faujasite, especially zeolite Y having a desirably high SiO.sub.2 /Al.sub.2 O.sub.3 ratio, will be crystallized under commercially viable production conditions.
Processes for producing commercially successful zeolitic cracking catalysts useful in moving bed and fluidized (FCC) cracking units utilize the concept of employing calcined clay reactant(a) in substantially the same size and shape as the desired catalyst product. Because the bodies are zeolitized directly without a separate binding step to composite zeolite and binder, the technology is frequently referred to as "in situ" processing. Reference is made to the following commonly assigned patents of Haden et al. as examples of such processes: U.S. Pat. Nos. 3,367,886; 3,367,887; 3,433,587; 3,503,900; 3,506,594; 3,647,718; 3,657,154; 3,663,165 and 3,932,268. In the processing described in some of the aforementioned patents, the preformed particles containing the reactive calcined clays also contain appreciable uncalcined (hydrous) kaolin clay. See, for example, U.S. Pat. Nos. 3,367,886; 3,367,887; 3,433,587; 3,503,900; and 3,506,594. In other processes, hydrous kaolin is absent. See, for example, U.S. Pat. Nos. 3,647,718; 3,657,154; 3,663,165 and 3,932,268.
One effect of including hydrated clay in the preformed reactants in in situ processes is that hydrated clay is present in the formed particles after the particles undergo crystallization. Another difference results from the fact that reactant particles that exclude hydrated kaolin undergo crystallization while the preformed particles are immersed in sodium hydroxide solution as an external aqueous phase. Under these conditions, silica is leached from the preformed reactant particles and a sodium silicate mother liquor is formed. The molar ratio of SiO.sub.2 /Al.sub.2 O.sub.3 in the crystallized microspheres is lower than the 2/1 SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the clay in the reactant particles. In contrast, when hydrated kaolin clay is a component of the preformed particles, sodium hydroxide is either initially present in or is transferred into the preformed particles which are immersed in oil during aging and crystallization. Under these conditions there is no leaching of silica from the preformed particles and the crystallized product has essentially the same SiO.sub.2 /Al.sub.2 O.sub.3 composition as the preformed reactant particles. Still another difference is that when particles consisting of calcined clay are immersed or suspended in sodium hydroxide solution, e.g., the procedures described in U.S. Pat. No. 3,647,718, a zeolite, identified by X-Ray analysis as zeolite B, tends to crystallize when the crystallization of zeolite Y reaches or approaches a maximum level. Consequently, preformed reactant particles that are converted to zeolitic particles by procedures in which hydrated kaolin clay is excluded are significantly different in chemical composition from particles in which hydrated clay is present.
When using spray dried microspheres containing kaolin clay calcined to undergo the exotherm and free from hydrated kaolin, the microspheres are mixed with a solution of sodium hydroxide to form a slurry, which is then aged, typically for 4-8 hours at 100.degree. F. and subsequently heated to crystallize a zeolite of the Y-type, typically by heating the aged slurry at about 180.degree. F. for 20 to 25 hours. Preparation of an FCC catalyst in this manner and using small amounts of metakaolin in the form of microspheres and a major amount of kaolin calcined to undergo the exotherm in the form of microspheres is described in U.S. Pat. No. 3,647,718. Similar use of metakaolin in the form of powder and kaolin calcined to undergo the exotherm in microspheres separate from the microspheres composed of metakaolin is described in U.S. Pat. No. 3,657,154. Criteria for selecting ratios of microspheres of metakaolin to microspheres of clay calcined to undergo exotherm are set forth in U.S. Pat. No. 3,647,718 at col. 6, lines 3 to 18. Silica originally in the microspheres is leached or extracted during the reaction, producing a sodium silicate mother liquor which is removed in whole or in part from the crystallized microspheres. The Na.sub.2 O/SiO.sub.2 molar ratio of this mother liquor is within the range of about 0.4 to about 0.6, most typically about 0.52. Hence, the material is generally referred to as "sodium disilicate". Crystallization is terminated when a desired amount of sodium zeolite Y is present in the crystallized microspheres. Typically about 20 to 25% zeolite Y is present. The microspheres usually also contain a small amount, typically less than about 5% as estimated from X-Ray patterns, of the zeolite known in the art as sodium zeolite B. The microspheres containing the mixture of sodium zeolite Y and a silica-depleted (alumina enriched) residue of calcined clay must be subjected to ion-exchange treatment to replace sodium ions with more desirable cations. Typically, ammonium ions or ammonium and rare earth ions are used in the exchange treatment(s). The zeolite Y component of the catalyst generally has a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio above 4.5 but the overall SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of the composite particles is generally about 1, e.g., from about 1.3 to about 1.0. Thus, the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of the catalyst product is roughly half of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the kaolin clay feed which is about 2.
The microspheres of calcined clay used as reactants generally have an average diameter of about 60 to 70 microns and contain a minimal amount, e.g., 3-5% weight percent, of particles finer than 20 microns, equivalent spherical diameter. Typically, the largest particles have a diameter of about 150 microns. The reason for restricting fines in the microspheres used as a reactant is that the microspheres retain substantially the same size and shape during aging and crystallization and the content of particles finer than about 20 microns in the finished crystallized catalyst product should be limited because they are difficult or impossible to retain in fluid catalytic cracking units. Furthermore, fines introduced with the reactants and/or generated during processing interfere with the operation of filtration equipment used in carrying out the ion-exchange treatment necesssary to convert the crystallized microspheres into catalytically active and selective particles.
When the foregoing production scheme is conducted on a commercial scale some fines (e.g., particles finer than about 20 microns equivalent spherical diameter) are generated during aging and/or crystallization. These fines contain about 20 to 40% sodium zeolite Y (as estimated by quantitative X-ray diffraction determinations). There is an indication that at least part of the zeolite Y in the fines results from a chemical reaction carried out in the aqueous phase (in contrast to zeolite that is present as a result of breakdown of crystallized microspheres). In carrying out experimental work that led to the present invention it was found that the fines typically also contain significantly more of the zeolite having the X-Ray pattern of zeolite B than is contained in the crystallized microspheres. For examples, typical minus 20 micron fines contain zeolite B in amounts (estimated from X-Ray) to be about 20 to 40% by weight. Also present is amorphous silica-alumina derived at least in part from calcined clay. A small amount of filter aid material (e.g. diatomaceous earth) is also present in fines from a commercial plant. The origin of such material will be explained subsequently.
The fines generated as a side stream are advantageously removed from the mainstream of crystallized microspheres before the crystallized microspheres undergo filtration to remove the mother liquor. Removal of the fines can be accomplished by passing the slurry of crystallized microspheres and mother liquor through one or more hydroclones before the slurry undergoes filtration. In the hydroclones grade material is discharged as the underflow effluent and is fed to the deliquoring filter. The overflow effluent from the primary hydroclones is combined with the filtrate from the deliquoring filter and run through a secondary hydroclone. The underflow again goes to the deliquoring filter and the overflow becomes the "unclarified" silicate. Mother liquor is separated from the fines for eventual concentration and sale. Such a concentrated mother liquor by-product typically contains about 15% by weight Na.sub.2 O, 29% by weight SiO.sub.2 and 0.1% Al.sub.2 O.sub.3, the balance being water. A conventional rotating drum filter precoated with a filter aid such as diatomaceous earth is used to remove the fines from the sodium disilicate liquor. The fines build up on the filter and are gradually scraped as a moist cake from the surface of the drum. The cake removed from the filter is also associated with entrained sodium disilicate solution, typically in amount corresponding to about 3-5% SiO.sub.2 (weight basis), based on the dry weight of the fines. The material removed from the filter has been handled in the past as waste material, creating a potential disposal problem.
Procedures for preparing fluid zeolitic cracking catalyst particles that involve mixing additional sodium silicate solution with zeolite crystals and spray drying the slurry to form microspheres are known. Reaction products containing synthetic faujasite and obtained from sodium hydroxide solution and a mixture of calcined kaolin clay are used in processes described in the following: U.S. Pat. No. 3,515,683; U.S. Pat. No. 3,451,948; and U.S. Pat. No. 3,458,454 all assigned to Air Products and Chemicals, Inc. In these processes the zeolite-containing reation products are ground before spray drying. A grinding step is also utilized when the feed to the spray dryer is obtained by reacting calcined clay and sodium hydroxide solution in the absence of hydrated clay. See U.S. Pat. No. 3,515,682, also assigned to Air Products and Chemicals, Inc.
An attempt by one of the coinventors in the subject application to bind the drum fines produced as a side stream of a commercial process for producing fluid cracking catalysts by in situ reaction of preformed microspheres consisting of calcined kaolin clay was successful when the processing simply involved spray drying and ion-exchange. The fines used in the tests are believed to have contained appreciable grade-size microspheres. Surprisingly, when hydroclones were installed in the plant and operated such as to further reduce the losses of grade-size microspheres in the fines, the same processing was unsuccessful. Products with acceptable attrition resistance could not be obtained. In some instances, the spray dried microspheres did not appear to be sufficiently hard to survive ion-exchange in commercial equipment. It is believed that the attrition-resistant grade size crystallized microspheres present in the drum fines obscured the problem that was encountered when substantially all of the solids in the spray dryer feed was 20 microns or finer. In unsuccessful attempts to solve the problem, the prior art practice of grinding before spray drying was adopted. Surprisingly, grinding did not solve the attrition-resistance problem. Furthermore, when a grinding step was used, the ion-exchanged products were unexpectedly less active in cracking gas oil feedstocks than products obtained from the same filter drum fines without a grinding step. These difficulties, and the failure of prior art techniques to solve them, are now attributed to differences in composition between the materials previously bonded with sodium silicate and the "drum fines" essentially free of grade microspheres. Furthermore, it now is recognized that the hardness standards for present day commercial cracking catalysts are more stringent than they were when the patents above-noted were granted.